I. Field of the Invention
The present invention relates to multiple access, spread spectrum communication systems and networks. More particularly, the present invention relates to resolving timing uncertainty in received access channel transmissions in a spread spectrum communication system.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as those used in code division multiple access (CDMA) communication systems provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. Such techniques are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters," and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, under the title "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The above-mentioned patents disclose multiple access communication systems in which a large number of generally mobile or remote system users each employ at least one transceiver to communicate with other system users or users of other connected systems, such as a public telephone switching network. The transceivers communicate through gateways and satellites, or terrestrial base stations (also sometimes referred to as cell-sites or cells).
Base stations cover cells, while satellites cover footprints or spots on the surface of the Earth. In either system, capacity gains can be achieved by sectoring, or subdividing, the geographical regions being covered. Cells can be divided into "sectors" by using directional antennas at the base station. Similarly, a satellite's footprint can be geographically divided into "beams," through the use of beam forming antenna systems. These techniques for subdividing a coverage region can be thought of as creating isolation using relative antenna directionality or space division multiplexing. In addition, provided there is available bandwidth, each of these subdivisions, either sectors or beams, can be assigned multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite systems, each CDMA channel is referred to as a "sub-beam," because there may be several of these per "beam."
In communication systems employing CDMA, separate links are used to transmit communication signals to and from a gateway or base station. A forward link refers to the base station- or gateway-to-user terminal communication link, with signals originating at the gateway or base station and being transmitted to a system user, or users. A reverse link refers to the user terminal-to-gateway or -base station communication link, with signals originating at a user terminal and being transmitted to the gateway or base station.
The reverse link is comprised of at least two separate channels: an access channel and a reverse traffic channel. An access channel is used by one or more user terminals, separated in time, to initiate or respond to communications from the gateway or base station. This communication process is referred to as an access transmission or as an "access probe." A reverse traffic channel is used for the transmission of user and signaling information from the user terminal to one or more gateways or base stations during a "call" or call setup. A structure or protocol for accesses channels, messages, and calls is illustrated in more detail in the Telecommunications Industry Association IS95 standard entitled "Mobile Station-Base-Station Compatibility Standard For Dual-Mode Wideband Spread Spectrum Cellular System," which is incorporated herein by reference.
In a typical spread-spectrum communication system, one or more preselected pseudo-noise (PN) code sequences are used to modulate or "spread" user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given cell, beam, or sub-beam.
In some communication systems, the same set of forward link PN spreading codes are also used in the reverse link, for both the reverse link traffic and the access channels. In other proposed communication systems, different sets of PN spreading codes are used between the forward link and the reverse link. In still other communication systems, different sets of PN spreading codes have been proposed for use between the reverse link traffic and access channels.
The PN spreading is accomplished using a pair of pseudonoise (PN) code sequences, or PN code pair, to modulate or "spread" information signals. Typically, one PN code sequence is used to modulate an in phase (I) channel while the other PN code sequence is used to modulate a quadrature phase (Q) channel. This PN modulation or encoding occurs before the information signals are modulated by a carrier signal and transmitted by the gateway or base station as communication signals on the forward link. The PN spreading codes are also sometimes referred to as short PN codes or sequences because they are relatively "short" when compared with other PN codes or code sequences used by the communication system.
A particular communication system may use several lengths of short PN code sequences depending on whether the forward link or the reverse link channels are being used. For the forward link, the short PN codes typically have a length from 21.sup.10 to 2.sup.15 chips. These short PN codes are used to distinguish between signals transferred by the various satellites, or gateways and base stations. In addition, timing offsets of a given short PN code are used to discriminate between beams of a particular satellite, or cells.
For the reverse link in a satellite system, the short PN codes have a sequence length on the order of 2.sup.8 chips. These short PN sequences are used to enable a gateway receiver to quickly search out user terminals that are trying to access the communication system without the complexity associated with the "longer" short PN codes used in the forward link. For purposes of this discussion, "short PN codes" refer to the short PN code sequences (2.sup.8 chips) used in the reverse link.
Another PN code sequence, referred to as a channelizing code, is used to discriminate between communication signals transmitted by different user terminals within a cell or sub-beam. The PN channelizing codes are also referred to as long codes because they are relatively "long" when compared with other PN codes used by the communication system. The long PN code typically has a length on the order of 2.sup.42 chips. Typically, an access message is modulated by the long PN code, or a specific "masked" version of such a code, prior to being modulated by the short PN code and subsequently transmitted as an access probe to the gateway or base station. However, the short PN code and the long PN code may also be combined prior to modulating an access message.
When a receiver at the gateway or base station receives an access probe, the receiver must despread the access probe to obtain the access message. This is accomplished by forming hypotheses, or guesses, as to which long PN codes and which short PN code pair modulated the received access message. A correlation between a given hypothesis and the access probe is generated to determine which hypothesis is the best estimate for the access probe. The hypothesis that produces the greatest correlation, generally relative to a predetermined threshold, is the selected hypothesis. Once the appropriate hypothesis is determined, the access probe is despread using the selected hypothesis to obtain the access message.
This timing uncertainty poses a problem for spread spectrum communication systems. This timing uncertainty corresponds to an uncertainty in the start of the PN code sequences, that is the starting point or timing of the code. As the timing uncertainty increases, more hypotheses have to be formed to determine the start of the PN code sequences. Proper demodulation of signals in these communication systems hinges on "knowing" where the various PN code sequences start in the received signal. Failure to recognize the start of the PN code sequences, or properly synchronize to their respective timing, results in failure to demodulate the received signal.
However, in satellite communication systems an access probe is particularly difficult to acquire, due to the changing distance between the user terminal and the satellite repeater. As the satellite orbits the Earth, the distance between the user terminal and the satellite varies considerably. The maximum distance occurs when the satellite is located at a horizon with respect to the user terminal. The minimum distance occurs when the satellite is located directly overhead of the user terminal. This variance in the distance creates an uncertainty in the one-way (i.e., from the user terminal to the gateway) timing of the access probe of up to 20 milliseconds (ms). Depending on the system, this uncertainty could be much more.
In order to resolve the timing uncertainty, the gateway receiver may have to search tens of thousands of hypotheses. This search may take several seconds to accomplish, resulting in a delay in establishing a communication link that is unacceptable to the user. Furthermore, due to the limited number of channels in the communication system, a particular user may actually lose an opportunity to access the communication system for several minutes because one or more other users establish a link or call first.
A similar situation arises in communication systems that employ a slotted ALOHA access signal protocol or technique. In this technique, the access channel is divided into a series of fixed length frames or time slots used for receiving signals. The access signals are generally structured as "packets", that consist of a preamble and a message portion, that must arrive at the beginning of a time slot to be acquired. A failure to acquire an access probe during a particular frame period results in the transmitter desiring access having to re-send the access probe to allow the receiver to detect the probe again during a subsequent frame. Multiple access signals arriving together "collide" and are not acquired, requiring both to be resent. In either case, the timing of subsequent access transmissions when the initial attempt fails is based on a delay time equal to a random number of time slots or frames. The length of the delay in probe acquisition is increased by any delay in resetting acquisition circuits in the receiver to scan the various hypothesis, and in other probes being acquired first, as mentioned Ultimately, the access probe may never, at least not within a practical time limit, be acquired if the timing uncertainty is not resolved.
What is needed is a system and method for rapidly acquiring the access probe in spread spectrum communication systems, in the presence of anticipated timing uncertainties.